


The Sleepover

by SoloShikigami



Category: Undertale (Video Game)
Genre: Canon Lesbian Relationship, Dare, F/F, Gay Sex, Lesbian Sex, M/M, Oral Sex, Sibling Incest, Sleepovers, Voyeurism
Language: English
Status: Completed
Published: 2016-10-24
Updated: 2016-10-24
Packaged: 2018-08-24 08:33:07
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,520
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8365273
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: Sans, Papyrus, Alphys, and Undyne have a thing with sleepovers; they watch anime, eat all sorts of junk food, and just in general have a good time. But when Undyne gets bored, she issues challenges, and she didn't think the two skeletons would entertain her idea of a challenge... but Alphys is ready to play along as well.





	

**Author's Note:**

  * For [Spoopygaster](https://archiveofourown.org/gifts?recipient=Spoopygaster).



It all started with their weekly sleepover – a tradition they had started since they came to the surface. They had taken the coffee table away from the middle on the L-shaped couch and filled the now-empty area with blankets, sleeping bags, and pillow to create a comfortable place for all four of them to sit, lay down, and cuddle as they watched anime.

But tonight, the television was off.

Undyne sighed. "I can't believe I could actually get bored up here."

"HOW COULD YOU POSSIBLY BE BORED?" Papyrus asked. "THERE ARE PLENTY MORE ANIME SERIES THAT WE COULD WATCH, LOTS OF BOARD GAMES AT OUR DISPOSAL, OR I COULD RE-ENACT MY DARING RESCUE OF THE HUMAN CHILD'S BLANKIE FROM THE CLUTCHES OF GERTRUDE THE GRUMPY GOAT!"

Sans chuckled, as he had heard the story earlier that afternoon; Papyrus found a job as a guard in the local petting zoo and the resident old goat began to nibble on a toddler's security blanket. After pulling the blanket free of the goat, it had chased Papyrus around the pen.

"Please, don't _goat_ him on," Sans said with his trademark grin. " _Ewe_ 'll be sorry you did."

Undyne and Papyrus both glared while Alphys hid a giggle behind her hand.

"For that, you owe us entertainment," Undyne said.

"Sure, I have no prob- _llama_ with that. Any particular requests?"

"OH, CAN I HELP? THE GREAT PAPYRUS IS ALSO A GREAT ENTERTAINER!"

Undyne's scowl deepened for a moment, but then she relaxed into a smirk.

"How about you and Papyrus give us a little show?" she asked in a slow, deliberate manner. "Like how they do in the anime."

Sans blinked. "You want me to fight giant robots in a sailor dress?"

Alphys outright snorted with laughter at the mental image.

"No, like what the twins do in Ouran."

Alphys immediately stopped laughing and her face turned completely red. Sans's face barely changed from his neutral, if not somewhat amused, expression.

Papyrus, however, was borderline ready to throw a fit.

"UNDYNE! THAT'S... INDECENT!" Papyrus cried.

"What? You should have stopped Sans from saying his stupid puns!" Undyne said.

Alphys grabbed her pillow and clamped it over her head.

Papyrus continued to sputter, only falling silent when familiar fingers traced up his femur, pressing the soft material of his navy blue pajama pants against his bones.

"Aw, come on, Paps, can't be that bad," Sans said softly.

Papyrus went to protest, but he could only shudder as the touch on his leg trailed higher and Sans moved closer.

"Think of it as the chance to cosplay."

Sans' hand moved to the inside of his femur, enjoying the flustered look on Papyrus' face as he struggled between being indignant and the embarrassment of being touched so intimately in front of others.

"B-b-but, Sans," Papyrus murmured, unable to look anywhere but where Sans' hand lay rubbing one of his sensitive spots. "They're our friends."

"That makes them the least judgmental audience we could ever hope for," Sans said softy, moving even closer so his legs pressed against Papyrus'.

Papyrus glanced over to the other two; Undyne was watching pretty intently while Alphys at least had the decency to blush as she peeked out from under her pillow. An odd feeling stirred inside of him and he couldn't tell if it was because the two girls were watching them so intently, or if it was because Sans had reached up to caress the side of his face before turning his attention away from their audience to kiss him gently.

Sans' body gave a little shiver of want as Papyrus pressed back into the kiss, delighted that he was going to play along. He carefully encouraged Papyrus to shift to the side, giving the girls a better view, as he climbed into his lap, hands cupping his face as he took complete control and deepened the kiss.

"God damn," Undyne swore in a murmur, leaning towards Alphys. "I was kind of joking; I didn't think they would actually do it."

Alphys wasn't clutching the pillow so tightly over her head anymore and watched the two a bit more openly. The blush was still there and it deepened when Sans spared a small glance and a knowing smirk in her direction before returning his attentions to his brother.

"It is kind of... Sexy," Alphys said softly.

Undyne nudged her with an elbow and a grin.

Papyrus was vaguely aware of the girls chatting, but their words only joined the buzzing in his head as Sans kissed him slowly, deeply, his blue tongue slowly forming and moving inside of his mouth while his own formed to tangle with it. He drew Papyrus' tongue into his own mouth for a bit before pulling back slowly, scraping his teeth lightly across the pale orange surface.

“S-Sans,” Papyrus breathed. “W-we shouldn’t do this.”

Sans pressed in closer. “Oh, I think we should,” he licked up his neck, his hands pressing over his ribs through his pajama shirt. “You know, it’s impolite to not honor the request of a lady.”

Papyrus pressed back into the side of the couch with a whimper, trying back away from Sans, his hands gripped the sheets underneath him.

“N-no, S-Sans, we r-really shouldn’t,” Papyrus murmured.

Sans stopped. “Do you really want to? I’m sorry.”

Papyrus let out a small laugh. “No, I thought you wanted to do like in the anime.”

Sans chuckled. “You’re so cute.”

“Nyeh, heh, heh,” Papyrus chuckled, loud enough for the girls to hear. “Should I call you _aniki_?”

“Mmm, you can call me anything you want. Though I plan to make you speechless.”

Sans returned to licking at Papyrus’ neck, nibbling now and then as he continued to squirm underneath him. Papyrus leaned his head back to give Sans more room as his hands found their way under Sans’ nightshirt and began to stroke his spine.

“This is… actually pretty hot,” Undyne murmured, scooting a little closer to Alphys.

Alphys was still a blushing mess, the pillow had fallen off of her head, but she didn’t care.

Sans moved back to capture Papyrus’ mouth in another deep kiss, moaning as the long fingers traced over a particularly sensitive vertebrae and stroked at the disc just above it. Papyrus wrapped his other arm around Sans, holding him tightly. Sans’ hands shook slightly as he started unbuttoning Papyrus’ night shirt. Papyrus broke the kiss, sitting back again and watching Sans’ fingers work, shuddering ever so slightly underneath them.

Undyne shifted a little; she was beginning to feel a little uncomfortable, her mind being drawn to doing some of the same things to the little yellow lizard sitting next to her. She startled when she felt a warm hand on her thigh and her eyes shot to Alphys. Alphys’ eyes were still trained on the two in front of them and she tended to get a little touchy feely when the anime they were watching turned up the heat factor, so, for now, Undyne tried to ignore it.

Sans pushed aside the soft material, fingers brushing across the slender ribs, his breath catching slightly.

“I will never get over how beautiful you are,” Sans said, one hand resting over Papyrus’ sternum while the other fingered the floating ribs.

Papyrus blushed and turned his face away; he was always pleased to be complimented, but in their current situation it felt a touch embarrassing. He could help chancing a glance up at his friends. Undyne had a blush of her own creeping over her cheeks and Papyrus was wondering if it was because she was watching one of her closest friends being intimate with his lover, or that Alphys’ hand was wandering closer and closer to her inner thigh. He locked eyes with Undyne for a moment, giving her a small, meaningful smile before turning his attention back to Sans and his wandering hands.

“Mmm, Sans,” Papyrus breathed as the other leaned down to lick a line up the center of his sternum. Then Papyrus gasped as Sans scratched lightly at his ribs. “S-Sans!”

“Surprise, little bro,” Sans said with a chuckle before scrapping his fingertips against the ribs again, a little slower, drawing a low moan from Papyrus and making him squirm again. “Hmm, I think he likes it.”

“P-p-please, Sans,” Papyrus shuddered, stuck once again between wanting to back away and wanting to push into the other.

“Oh, man, this is nuts,” Undyne murmured. “How far do you think they’ll take it?”

“How far do you want them to take it?” Alphys asked.

Undyne opened her mouth to answer, but then it snapped shut when a pair of fingers brushed across her crotch. She turned her head towards Alphys almost fast enough to hurt herself and stared blankly at the normally shy and submissive monster. She was still blushing, but a small smile turned one corner of her mouth.

“You are such a dirty girl,” Undyne murmured, shifting herself a little closer to her girlfriend. “I think I like this.”

Alphys’ smile grew just a fraction.

By the time the girls returned their attention to the skeletons, Papyrus was a quivering mess under Sans’ hands. His tongue hung out of the corner of his mouth, a small bit of drool painting an orange trail down his jaw.

Sans chuckled and rocked his pelvis against him. “You seem to be having fun, Papyrus.”

Papyrus could only groan.

“Heh, maybe I got your goat, eh?”

Papyrus looked up at Sans. “You didn’t just-?”

“Oh, yes I did,” Sans leaned down and licked the bridge if Papyrus’ nasal bone. “What are you going to do about it?”

Papyrus’ face went dark and Sans backed off a bit, wondering briefly if he took it too far.

The girls jumped slightly at the sudden movement as a moment later, Papyrus had Sans pinned underneath him. He knelt between Sans’ legs, one hand reaching underneath the nightshirt and pressing on his sternum while the other hand pressed against his boxers.

“P-Papyrus?” Sans stammered, not expecting the sudden shift in dynamic. “Pap, hey, I’m sor-gaaaah!”

The hand on Sans’ boxers rubbed him purposefully. Papyrus chuckled as he leaned over him.

“You wanted to give the girls a show, and I cooperated, I even tried to do the pretty boy blushing and denial,” Papyrus said just loud enough for the girls to hear. “And how do you repay me? With one of your horrid puns? Nyeh, heh, heh, well, dear brother, the Great Papyrus simply won’t allow that to stand.”

Sans’ bones rattled a bit; Papyrus taking the lead was a turn on to begin with, but this was a new level and he was willing to go where ever he would lead.

“Oh, damn, that’s hot,” Undyne murmured.

“Yeah, I can tell,” Alphys said with a certain knowing lilt in her voice.

“What’s the supposed to mean?”

Undyne sucked in a breath when Alphys pressed her hand into her crotch a little harder. Alphys turned to her so she could whisper in her ear.

“You’re already wet.”

Undyne shuddered. “Y-yeah,” she felt her face heat up.

“Well, I’m not going to let it go to waste.”

Undyne’s eyes widened as Alphys’ hand trailed to the waistband of her soft, cotton shorts, pulling on the elastic band a little so her hand could slide inside.

Sans’ moan grabbed their attention again. Papyrus had gripped Sans’ wrists and easily held them pinned above his head while his other hand continued to work the front of his boxers. Sans writhed underneath his brother, trying and failing to gain control over anything he could. Papyrus’ fingers soon felt Sans’ magic forming under his shorts, no longer leaving it completely up to the girls’ imaginations of what was going to happen.

“Oh, my,” Alphys murmured, her eyes glued to the display before her. “This is better than any anime.”

Undyne could only nod, then she let out a small moan of her own when she felt Alphys’ fingers brush over her vulva without anything acting as a barrier. Undyne slapped her hand over her mouth when she moaned again when Alphys’ repeated the action. Alphys smiled and moved so she was sitting behind Undyne, still able to see the skeleton brothers, keeping the one hand firmly between Undyne’s legs while the other wrapped around her torso.

“Nyeh heh heh, looks like we inspired the girls to do some cosplaying of their own,” Papyrus murmured, leaning down to kiss Sans.

Sans offered himself freely to Papyrus, letting him drive all of it. Alphys and Undyne’s presence was a far off thing in his mind as Papyrus worked his mouth as well as his forming dick. He glanced over at their friends, noticing Alphys was likely fingering Undyne underneath her shorts, and her other hand had sneaked up her shirt and was obviously caressing her breasts. Sans moaned, pressing up into Papyrus’ mouth, then whined when Papyrus pulled away.

“Ready to take it to the next level?” Papyrus said.

Sans nodded, panting and wriggling desperately against Papyrus’ hands. Papyrus grinned and began to pull down Sans’ boxers with one hand, freeing his fully formed cock.

Undyne was barely paying any attention; Alphys knew where and how to touch Undyne to drive her wild. She had slipped a pair of fingers into her folds and was soon slowly pumping her dripping hole. Her other hand was gently squeezing her breast, fingertips circling the nipple, loving how it fit perfectly in her hand.

“Oh, Undyne, I love you so much,” Alphys whispered in her ear before dropping a kiss on her shoulder.

“Alphys,” Undyne whined, leaning back into her arms. “Fuck, that feels good.”

Alphys smiled and kept going.

Papyrus was completely focused on his task, wrapping his hand around the blue cock and stroking it slowly. Sans gasped at the touch and hitched his hips up into it.

“Fuck, fuck, Paps, please, more,” Sans begged.

“Mmm, you’re so eager this evening,” Papyrus purred. “Is it because we have people watching us?”

“Guh, could be?” Sans panted.

“My, my, dear brother, how indecent,” Papyrus said, squeezing a little harder as he stroked, swiping his thumb across the head as he came up.

“F-f-fuck,” Sans turned his head into his shoulder with a small sob.

“Alphys!”

The two boys were distracted for a moment and looked over to Undyne as she slowly fell apart under Alphys’ touches.

“That’s my girl,” Alphys murmured between pressing kisses to the side of Undyne’s face, her neck, and her shoulders. “I got you.”

Papyrus’ hand slowed, but it hadn’t stopped. Sans could feel a coil of energy in his groin grow painfully tighter at the display. He moved his hips up, reminding Papyrus that he was in the middle of something.

Undyne gripped Alphys’ arms, her body beginning to shake, her breath coming in short gasps. Alphys didn’t stop and she didn’t speed up, enjoying the fact that she was the one making this strong monster, the one who could take down just about anyone, monster and human alike, and yet, Alphys, sweet, innocent-seeming, awkward little Alphys, was making bringing her to her metaphorical knees.

“Oh, Undyne, you’re so beautiful,” Alphys murmured.

Undyne tried to bite back the loud moan that came forth, but there was no stopping it. Her body shook violently as her orgasm crashed into her, the grip around her tightened, the fingers inside her slowing just slightly, easing her down.

“Papyrus,” Sans moaned as Alphys leaned forward to kiss Undyne.

“I got you,” Papyrus murmured, returning his attention to Sans.

The taller skeleton leaned down to kiss Sans again as his hand resumed the long strokes, moving a little faster to bring Sans back to where he was before they were distracted. Undyne and Alphys watched them intently again.

“You know, you need a turn, too,” Undyne murmured.

“Shh.” Alphys whispered.

“Mmm,” Sans broke their kiss. “Papyrus, please, I want you.”

“Oh really?” Papyrus murmured. “You want me to take you here and now with others watching us?”

Sans blushed. “Y-yes.”

“Oh? You want me to claim you in front of these ladies?”

Sans shut his eyes tightly. “Yes, Papyrus, please.”

“Even though they are our friends?” Papyrus drew out the last few words in a whisper that Sans barely caught.

Sans shuddered with a loud groan. “Yes, please, I need it, Papyrus please!”

“Oh, stars, please,” Alphys murmured, squeezing Undyne as if she were a pillow.

“You’re really getting into this,” Undyne murmured while Papyrus moved back to slowly push down his pajama pants.

Sans stayed where he was, wrists pressed into one another as if they had been tied there.

“So are you… Oh, my.”

Both girls blushed when they saw Papyrus’ member. Sans couldn’t help a small bit of satisfaction when he realized the two went silent.

Papyrus paid no attention at all; he was quite pent up himself and just as eager to be inside of Sans as San was to have him there. He stroked himself a few times before bending over Sans again.

“Are you sure about this?” Papyrus asked.

“Papyrus, I don’t think I’ve ever been so turned on in my life,” Sans admitted.

“Nyeh heh heh, all right, if it’ll please you.”

Sans was grateful that Papyrus was occupying his mouth as he thrust inside of him; the cry he wanted to released was way too embarrassing for an audience, but the cry quickly dissolved into low moans as Papyrus moved in slow, shallow thrusts.

“Oh, oh my, oh dear,” Alphys sounded like she was fretting, gripping onto Undyne tighter.

Undyne grinned just before she turned and tackled Alphys to the ground, kissing her deeply.

“So, that turns you on, huh?” Undyne asked, pressing into her. “Would you want to be fucked by a cock like that?”

“Undyne!” Alphys squeaked, covering her face as it turned red again. “U-Undyne, what are you-?!”

Undyne snickered as she pushed up Alphys’ nightgown. She smiled at the dark spot that formed on the black panties Alphys was wearing. Undyne wasn’t too good with words and always felt action spoke better, so she wasted no time pressing her face between Alphys’ legs, her tongue pressing on that wet spot and making the lizard monster gasp.

Papyrus picked up his pace and lengthened his thrusts as Sans loosened slightly around him.

“Oh, stars, Pap, that feel so good,” Sans murmured, his arms now wrapped around Papyrus’ neck, holding him so their foreheads touched.

“You like the way I feel inside of you?” Papyrus asked.

“Oh, you know I do, you’re so big, stretching me in all the right places, stars, you’re so perfect,” Sans murmured.

“Mmm, you feel good, too, Sans,” Papyrus murmured. “Nng, I, ah, I’m going to, soon.”

A small bit of Sans smiled at the small hint of innocence Papyrus always held onto.

“Yeah, me too,” Sans murmured.

He looked over at the girls and found Undyne’s head was under Alphys’ nightgown, her hands fondling her breasts through the soft fabric, as Alphys’ head tossed from side to side. Her eye caught Sans’ for a moment and Sans shuddered at the shared energy.

“Fuck, Papyrus, please,” Sans begged.

“Yes, brother, I need too as well,” Papyrus murmured, increasing his speed.

Sans didn’t have to wait long and his eyes widened in shock at how quick and how hard his orgasm hit him. His back arched as he tightened around Papyrus, which only pushed him off the edge as well. Sans wrapped his arms tightly around Papyrus, his insides milking his cock as the orgasm faded, their magic soon following suit. Papyrus panted for a moment, catching his breath before he kissed Sans.

“I love you,” Papyrus murmured.

“Love you too, bro,” Sans replied.

The two shifted a little, holding each other as they watched the girls.

“Guh, Undyne, please,” Alphys murmured.

Undyne grinned inwardly, her tongue pressing inside of Alphys, trying to mimic what she knew Alphys was thinking about, making a small mental note that she needed to find one of those adult toy stores she remembered hearing about. In the meantime, her tongue would more than suffice, for it knew where to press, how hard to thrust, how to curl inside of Alphys to bring her to her peak.

“Ah, Undyne, Undyne!” Alphys whined, her legs shaking as one hand gripped Undyne’s hand, intertwining their fingers while the other grabbed a nearby pillow, her body convulsing as her own orgasm took over.

Undyne gripped Alphys’ hand in return, lovingly lapping up her juices as they flowed, dropping a few kisses along her folds and her inner thighs, leaning up to drop a few kisses along her belly (one of her bodily features that Alphys’ tended to not like so much), before moving up to kiss her panting lips.

“Mmm, that was quite the show, girls,” Sans murmured.

“You two nerds didn’t do so bad yourself,” Undyne said with a smirk.

“Oh, stars, I can’t believe we just did that,” Alphys murmured.

“Me neither, wowie,” Papyrus said, squeezing Sans.

Needless to say, they weren’t bored any longer.

 


End file.
